My understanding is that actors don't typically have a choice whether to join a union. SAG-AFTRA has enough power and preference agreements that you can't really make it in the industry without joining, even if you don't otherwise want to.

Possibly, but it doesn't have the problem of treating the actors as a homogeneous equally skilled block; "10X actors" are definitely getting adequate compensation while at the same time all guild members benefit from the common protections.

You are mostly correct. You can do one off acting but any major production will only hire SAG artists